The present invention relates in general to the field of ion generation and in particular to a new and useful surface ionization source which is capable of converting neutral molecules into ions containing the molecules.
Many ways are already known for producing gas phase ions for analysis by mass spectrometry. The most commonly used of these methods are electron impact, in which a beam of energetic (typically 70 electron volts) electrons is passed through a low pressure (less than 10.sup.-4 torr) of vaporized sample. This was the earliest and is still the most widely used method of forming ions in commerically produced mass spectrometers. Typically, the electron impact ionization results in the formation of a large number of fragment ions and in many cases does not produce a significant abundance of intact molecular ions. The molecular ions are useful because they provide information on the molecular weight of of the sample molecules.
The more recent innovation of chemical ionization forms ions by means of a gas phase chemical reaction between ions produced from a reagent gas, present in large excess, and the neutral sample molecules. These ions are typically the sample molecule plus one proton, and the molecular weight of the original sample can be easily deduced by subtracting the mass of a proton, namely, 1.0007825, from the measured mass of the MH+ ion.
Many other methods of ionization exist but are less commonly used. Among these are ionization by vacuum ultraviolet photons, termed photoionization; ionization by very high electric fields, termed field ionization; and ionization by bombardment of the sample upon a solid surface with high energy beams of either ionic or neutral particles.
Mass spectrometers are useful in detecting contaminants in the air. One such application is in the field of chemical warfare where it is desired to determine whether a chemical is present on a battlefield. The application of mass spectroscopy however is complicated in the environment of a battlefield which would be expected to contain other substances that would not normally be found in clean air.
A man-portable field alarm system for chemical agents based on the principle of tandem mass spectrometry or mass spectrometry/mass spectrometry (MS/MS) would be very useful if such a device could be constructed. Such man-portable field alarm systems would be required to detect trace levels of chemical agents in the presence of large concentrations of complex interfering substances, such as diesel fuel smoke. Based on the results of several space exploration programs, a single, small, compact, man-portable, battery-operated mass spectrometer system could easily be produced that is capable of detecting trace levels of chemical agents in clean air. However, to detect chemical agents in the presence in high concentrations of interfering mixtures, a more sophisticated MS/MS system is required.
In an MS/MS detector, the first mass spectrometer separates all ions corresponding to the molecular weight of the chemical agent from the large variety of ions produced from the total mixture of substances in the air. A particular mass-selected ion beam is then fragmented, and the fragment ions separated according to their mass by the second mass spectrometer. Because chemical agents have fragmentation patterns different from those of common interfering substances, the presence of fragment ions characteristic of chemical agents can be used to detect their presence, even in complex mixtures.
Limitations to the portability of mass spectrometers are size, weight, and power consumption. An examination of commercially available, small mass spectrometers, which are principally residual gas analyzers, shows that by far the largest single limiting factor is power consumption--specifically the vacuum system and its associated pumps. Conventional vacuum pumps use two separate stages of pumping to maintain a vacuum against the ambient atmosphere. The first stage is either a diffusion pump or a turbomolecular pump. This in turn is backed in the second stage by a mechanical vacuum pump, usually called a forepump. Even if the newer miniature turbomolecular pumps are used, which are lightweight and low in power consumption, the smallest available mechanical pump will push the power requirement for the vacuum pumps alone to well over 500 watts. Even if the weight of these items could be tolerated, 500 watts is well outside the capabilities of any man-portable battery pack.
An alternative vacuum pump, called a triode pump, ionizes gas by a continuous DC discharge. Pumping occurs by the burial of ions in a titanium cathode as well as by chemical reactions of neutral gas molecules with sputtered titanium metal. In operation, the power consumed by these pumps is well under 1 watt (this is as long as the gas load remains small; otherwise power would be 10 to 40 W); the major determinant of their power consumption is thus the efficiency of their required high-voltage power supply. Furthermore, once started, these pumps do not require backing by a mechanical forepump.
In use, the entire vacuum system, including the triode pump, is evacuated, baked and then sealed off. As long as the gas load on the vacuum system stays within the capacity rating of the triode pump, these devices can maintain a high vacuum without any mechanical pump requirements. Small triode pumps suitable for a portable mass spectrometer system have pumping speeds of 1-5 1/s and throughputs at low power consumption of up to about 10.sup.-4 Torr 1/s. The limited pumping capacity available means that ionization methods such as chemical ionization (CI) (not necessarily true in the case of the Finnigan Ion trap however), and atmospheric pressure ionization (API), both of which introduce large gas loads, would not be feasible for a man-portable mass spectrometer system. Commercially available systems would therefore not be practical for the portable field alarm detector.